It is well known that in industries such as high-tech electronics and pharmaceutical manufacturing it is necessary to prevent contamination by small particles. This need has given rise to the development of optical particle counters in which fluid flows through an optical sensor which detects the presence of contaminant particles. Particle counters which detect particles in air or other gas are sometimes referred to as “aerosol particle counters”. Standards generally require that aerosol particle counters monitor the entire volume of gas flowing through the sensor. Particle counters that monitor the entire volume of gas flowing through the sensor are referred to in the micro-contamination industry as “volumetric”. The micro-contamination industry has evolved two distinct methodologies for continuous volumetric aerosol particulate monitoring of environments. One method involves the use of an aerosol manifold system. The use of a manifold system allows a single particle counter to sequentially monitor multiple sample locations in an environment. This allows the cost of the particle counter, the manifold, and the system vacuum pump to be shared among multiple sample points. A major drawback of this method is the transport loss of large particles down the required lengthy sample tubing runs. This is unavoidable, as all sample points must be routed to the single particle counter. Some sample lines may reach over a hundred feet in length. Another major disadvantage of this method is the sample points can only be evaluated one at a time; therefore, real-time monitoring of all sample locations is not possible.
The second common methodology requires the use of dedicated particle counters at each of the required sample points. This method provides continuous real-time monitoring of all sample locations and also eliminates large particle transport tubing loss, as a particle sensor can be placed at or near each required sample location, thus minimizing sample tubing length. However, this method requires a large number of particle counters and, for large plants, can be quite expensive. The response to this problem has led to the development of particle sensors. A particle sensor generally will have no external display, keyboard, internal airflow pump, or variable flow control devices. This minimizes the cost of the sensor, but requires the end-user to provide an external vacuum pumping system. These sensors are most commonly 1.0 CFM (cubic feet per minute) or 0.1 CFM flow rate. With no internal airflow pump or variable flow control and measurement system, particle sensors generally utilize a critical flow orifice to control the sample flow rate. The use of a critical flow orifice to control volumetric flow rate has been well established as explained in Willeke/Baron, “Aerosol Measurement” and Hinds, “Aerosol Technology”. The required critical pressure drop needed is given by the following equation:PV/Pa=[2/(k+1)]k/k−1 where
PV=Pressure on vacuum side of critical flow orifice;
Pa=Pressure on upstream side of critical flow orifice;
K=Gas specific heat ratio=7/5 for diatomic gases=1.4.
Substituting 1.4 for k yields the simplified equation:PV/Pa=0.53.At standard conditions, Pa=14.7 psi. Therefore, the required critical pressure drop at standard conditions is 7.791 psi (15.9″ Hg). Measured in inches of water, this is 215.6 inches. That is, a pressure of 1 inch of water is equal to 7.791/215.6 psi or 0.0361 psi. Since, 1 psi is equal to 6,894.76 pascals (Pa). 1 inch of water is equal to 249.15 Pa or 0.24915 kPa, 2 inches of water is equal to 0.4983 kPa, and 3 inches of water is equal to 0.74745 kPa.
At standard conditions, a critical flow orifice will maintain constant volumetric flow when the downstream vacuum level is greater than 15.9 inches Hg. Under these conditions, the velocity in the throat of the orifice is the speed of sound, and a further increase in the downstream vacuum level does not increase the velocity through the throat. This requires the user to provide a vacuum pumping system that can maintain a minimum of 15 inches to 17 inches Hg vacuum level at the particle sensor's specified flow rate.
The requirement for a vacuum pump capable of maintaining >15 inches Hg for even a single 1.0 CFM particle sensor limits the available options of pump choice to a positive displacement pump such as a carbon vain rotary design. These pumps are quite large and consume greater than 100 Watts of power. As the user will typically install multiple particle sensors, the number of particle sensors in use will increase the pumping system requirements. It is quite common for the pumping system to weigh several hundred pounds and have power consumption measured into the thousands of watts. In addition, vacuum lines must be installed to run from the central vacuum pump to each and every sensor installed into the system. The final flow system install cost is typically $500 to $700 per instrument.
In recent years, particle counter manufacturers have begun to offer particle sensors that include internal pumps or blowers that are controlled by a closed loop flow measurement and control device. With the addition of an intelligent flow control system, the critical flow orifice may be removed. This eliminates the dominant pressure drop in the system and leaves the pressure drop of the particle sensor itself in addition to any desired inlet tubing. Current particle sensor pressure drops range from approximately 3 inches to 50 inches of water. Typical inlet tubing pressure drop is 2 inches to 10 inches of water for inlet tubing lengths reaching up to 5 feet in length. The total system pressure drop is then typically 5 inches to 60 inches of water, which includes the sensor pressure drop plus the inlet tubing pressure drop.
With the maximum pressure drop now significantly lowered from the 215.6 inches of water of the critical flow orifice designs, it is possible to utilize air-moving devices other than positive displacement pumps. Radial and regenerative blowers are capable of moving air in the pressure ranges reaching up to 25 inches of water. These blowers can be controlled with a DC voltage, making them easy to integrate into a closed loop flow control system. Typically, they operate in the 25 W to 75 W power range, making them a lower power option than positive displacement pumps. See U.S. Pat. No. 6,167,107 issued Dec. 26, 2000. The major disadvantage of radial and regenerative blowers is their cost. As with particle sensors that utilize critical flow orifices, the final system install cost remains at $500 to $700 per sensor. Thus, while these systems provide continuous monitoring and do not require a massive airflow system, still there is no cost advantage to utilizing these types of blowers in a multiple particle sensor system.
Thus, there remains a need of a system and method of controlling volumetric flow rate in an aerosol particle sensor that would substantially reduce the final installation cost of a multiple sensor system and yet provide continuous monitoring at all critical locations in an environment.